1. Field of the Invention
This invention relates, in general, to photocopying apparatus and, more specifically, to image transfer sequences used in such apparatus, especially when color transparencies are desired.
2. Description of the Prior Art
Electrostatographic apparatus such as copiers, printers, duplicators, and like devices, can be used to provide hardcopy output sheets containing image information placed on the sheets by the apparatus. In some applications, it is desirable to interleave the output sheets with other sheets for various purposes. Interleaving is especially useful when the output sheets consist of a transparency material.
U.S. Pat. No. 4,681,428, issued on Jul. 21, 1987 with the same assignee as the present invention, describes apparatus for producing interleaved copy sheets. The background section of that patent describes some of the advantages and uses of the interleaved sheets. In general, interleaved sheets reduce the tendency of the sheets of transparency material to stick together and also allows them to slide more freely when a uniform stack is desired. Other uses of interleaved sheets include the ability to produce a copy of what is on the transparency which can be examined or referred to without the need for special projection apparatus and the cleaning action on the fuser to remove contaminants or release agents which might degrade the appearance of the transparency.
The method used to interleave the extra sheets with the output sheets depends upon the type of equipment involved. One type of apparatus presently being used has a "two-up" transfer drum which can handle transfers in quick succession to two output sheets located on the transfer drum substantially at the same time when the sheets are of a specific size. U.S. Pat. No. 4,712,906, which is also assigned to the same assignee as the present invention, discloses electrostatographic apparatus which has a two-up transfer drum. While such apparatus is also useful when making non-color copies, the throughput enhancement provided by the two-up drum is especially important in color copy apparatus where the throughput speed is significantly lowered by the color process.
The apparatus shown in the 4,712,906 patent employs a direct transfer member wherein the output sheets are positioned on the transfer member and receive the developed images directly from the photosensitive member of the machine. Other apparatus known in the prior art uses intermediate transfer drums, rollers, or belts to make the developed image transfer between the photosensitive member and the output sheet. In such apparatus, the developed image is first transferred to the intermediate transfer member and then transferred to the output sheet from the intermediate transfer member. When color copies are to be made, the component color images are all transferred in registration to the intermediate transfer member before the final or composite color image is transferred in one operation to the output sheet.
Whether using direct or indirect transfer apparatus, providing interleaving sheets with two-up transfer devices has the tendency to considerably slow down the throughput rate when color outputs are being produced. According to the prior art, only one of the two image or sheet areas on the two-up transfer member is allocated to the images being reproduced on the regular output sheets. The other area is used to allow integration of the interleaved sheet into the paper flow path of the apparatus. Thus, a throughput rate of only 50% the normal rate can be achieved with interleaved sheets according to such an operational sequence.
Because production speed is important and it increases the efficiency and desirability of the apparatus, it is desirable, and it is an object of this invention, to provide apparatus and methods which allow the interleaving of output sheets with a minimum of degradation of throughput speed.